12 + log (N/H) vs. 12 + log (He/H) (upper panel) and log (N/O) vs. 12 + log (O/H) (lower panel) for PNe (in black) in NGC 6822. In red the H ii regions. Stellar evolution models for different metallicities by Karakas (2010, K10) (Z = 0.0001, blue diamonds; Z = 0.004, magenta triangles) and Fishlock et al. (2014, FK14) (Z = 0.001, green squares) have been included. Masses of the progenitors stars are shown for some models. See text for discussion.
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